Electrical logging system for use with a drill string

ABSTRACT

An electrical logging system adapted for installation on a drill string for use while drilling. The system includes a specific electrode arrangement that will operate in the vicinity of ferromagnetic drill strings. The electrode arrangement may also be used with more conventional wireline resistivity logging systems.

iluited Stat Vogel ELECTRICAL LOGGING SYSTEM FOR USE WITH A DRILL STRING[75] Inventor: Charles B. Vogel, Houston, Tex.

[73] Assignee: Shell Oil Company, New York, N.Y.

[22] Filed: June 21, 1971 [21] Appl. No.: 155,028

[52] 11.8. C1. 324/10, 324/1 [51] Int. Cl. Glv 3/06, GOlv 3/18 [58]Field 01 Search 324/1, 10, 6, 5

[56] References Cited UNITED STATES PATENTS 3,134,069 5/1964 Clements eta1. 324/ 3,488,574 1/1970 Tanguy 324/10 X 1 July24,1973

2,941,784 6/1960 Martin 324/10 2,917,704 12/1959 2,313,384 3/19432,018,080 10/ I935 Martienssen 324/5 Primary ExaminerGerard R. StreckerAttorney -Theodore E. Bieber et al.

[57] ABSTRACT An electrical logging system adapted for installation on adrill string for use while drilling. The system includes a specificelectrode arrangement that will operate in the vicinity of ferromagneticdrill strings. The electrode arrangement may also be used with moreconventional wireline resistivity logging systems 5 Claims, 3 DrawingFigures 52 FOW FROM SURFACE SUPP/y MULT/PLEX I A D TO SURFACE 76 lDIGITAL COVDE 57 FROM 5 URFA CE MUL Tl PL EX A ND PATENTED Jill 2 4 I973SUPPLY ELECTRICAL LOGGING SYSTEM FOR USE WITH A DRILL STRING BACKGROUNDOF THE INVENTION The present invention relates to logging devices andmore particularly to an instrument assembly for use in electricalresistivity logging. In electrical resistivity logging, it isconventional to provide a source electrode for inducing a current flowin a vertical direction in the earth formations surrounding a boreholeand two detecting electrodes for measuring the resistivity of theformation. The detecting electrodes are spaced various distances apartto measure the resistivity over long or short intervals. With thesearrangements, the measurements are affected by alterations of the earthin the vicinity of the hole as a result of the drilling process. Byusing electromagnetic induction, devices have been made which areunaffected by material in the immediate vicinity of the hole, but theseemploy currents flowing in hroizontal directions, whereas theresistivity for vertical currents is often desired. Moreover, existingdevices will not operate satisfactorily when a steel pipe is within theborehole, except by use of fragile toroidal coils.

In addition to the above problems, there is also the problem ofinstalling and operating an electrical logging system in a drill string.Many systems have been designed for telemetering data from the downholedrill string to the surface while drilling but no electrode system hasbeen designed. For example, U.S. Pat. Nos. 3,518,608 and 3,518,609described telemetering systems wherein a single conductor cable ispositioned along the interior of each joint of the drill string andterminating in suitable electrical fittings at the'ends of each joint.The electrical fittings automatically complete the circuit when theindividual joints are assembled.

While circuits are available for telemetering information to thesurface, suitable downhole instrumentation has not been developed. Oneproblem in developing downhole electrical logging instrumentation forinstalling in a drill string is the interaction of the magnetic fieldproduced by solenoid coil type sensors with the material of theferromagnetic drill string. This interaction is of sufficient magnitudeto substantially mask voltages induced by currents in the earth,preventing successful resistivity logging of the borehole while drillingby induction methods. Also, the high conductivity of the drill stringprecluded use of conventional electrode systems unless the exterior ofthe drill string is insulated by rubber or otherwise.

SUMMARY OF THE INVENTION The present invention solves the above problemsby providing a downhole electrical logging instrumentation forinstallation on a drill string. The instrument includes a pair of sourceelectrodes positioned on drill string to force a current flow in thedrill string and a pair of detector electrodes are also positioned onthe drill string below the source electrodes. One of the electrodes ofeach pair is coupled to the remaining portion of the instrument by aconductor positioned on the outer surface of the drll string. The pairof source electrodes are coupled to a power supply capable of supplyingin a high frequency, preferable exceeding Kilohertz high amperagesignal. This causes the two segments of the drill string between thepairs of electrodes to serve as resistors having a resistanceconsiderably larger than the resistance of the drill string at lowerfrequencies. The upper pair of electrodes produce a significant voltagedrop and effectively create an electric dipole.

While the above description relates to the mounting of the system on adrill string, it is preferable that the electrodes be mounted on thedrill collar immediately adjacent to the drill bit. Thus the system willbe capable of accurately measuring the resistivity of the formationsthat are immediately beneath the bit and not yet drilled. When thisconfiguration is used, the current from the dipole formed by the upperpair of electrodes will flow downwardly through the drill bit into theearth. The amount of current flow will, of course, be related directlyto the resistivity of the formations below the bit. Thus, the voltagedrop across the two lower detector electrodes can be measured and willbe related to the reciprocal of the formation resistivity.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be moreeasily understood from the following detailed description when taken inconjunction with the attached drawings in which:

FIG. 1 is a vertical elevation view of the invention ap plied to a drillstring;

FIG. 2 is a cross-section taken along Line 22, FIG. 1, and

FIG. 3 is a vertical elevation of a logging tool incorporating theinvention.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring to FIGS. 1 and 2, thereis shown the electrical logging system of the present inventioninstalled on the lower drill collar of the drill string. The drillcollar 1 l is shown disposed in a borehole l0 and having a drill bit 12attached to its lower end. Drill collars, of course, are conventionalequipment used in the rotary drilling of oil wells'and the like. A firstpair of electrodes 13 and 14 are mounted on the upper portion of thedrill collar. As shown, the electrode 13 is mounted on the outside ofthe drill collar and the electrode 14 on the inside although both couldbe mounted on the inside or the outside. The electrodes should be spacedapproximately 24 inches on an 8 inch diameter drill collar. Theelectrode 14 is shown connected to the power supply 16 by a conductor 15that is disposed on the outer surface of the drill collar. The conductormust remain on the outer surface of the collar at least for the distancebetween the two electrodes before the conductor passes through apressure seal 17 to the interior of the drill collar. The power supplycoupled to the electrodes 13 and 14 produces a 30 kHz signal of 30amperes resulting in a small voltage drop. The power supply is poweredby electrical energy transmitted down the borehole by means of thetelemetering circuit described in the above-referenced patents. Ofcourse, the power supply 16 is actually disposed in the drill collar lleither by forming a recess in the drill collar or including the powersupply in the form of a package which may be attached to the drillcollar and project into the interior thereof.

The pair of electrodes 13 and 14 form an impedance having a value aboutone thousand times greater than that measured at lower frequencies. Thiseffect is enhanced by an increase in the magnetic properties of thedrill collar and since most drill collars are steel, they will havesufficient magnetic properties. The electrical current flows through thecollar by way of the pair of electrodes 13 and 14 and produces asignificant voltage drop and effectively creates a 60 millivolt dipoleof 2 ft. length. The current from the dipole flows through the lowerportion of the drill collar and into the earth, returning to the upperportion of the collar and drill string.

A second pair of electrodes, 30 and 31, are positioned to detect thevoltage drop in the drill collar. The electrode 30 is positioned on theinterior surface of the drill collar while the electrode 31 is mountedbelow the electrode 30 and on the outer surface of the collar. The twoelectrodes thus form a pair of electrodes for detecting the voltage dropin the drill collar which, in turn, can be related to the resistivity ofthe formation. Electrode 31 is coupled by means of a conductor 32 whichis disposed on the outer surface of the drill collar at least until itreaches a position where it is above position of the electrode 30 on theinterior of the drill collar. The conductor 32 may be positioned on theouter surface of the drill collar or may be placed in a grove 33 formedin the outer surface of the drill collar in order to protect it frommechanical damage. At the top the conductor passes through a pressureseal 34 into the interior of the drill collar. The leads from theelectrodes 30 and 31 can then be coupled to a multiplexer and digitalencoding system 50. The multiplexer and digital encoding system convertsthe measured voltage drop to a digital signal which can then betransmitted to surface over a lead 51 which is coupled to thetelemetering circuit described in the above patents. The use ofamultiplexer circuit is required in order to transmit more than onemeasurement over the single conductor telemetering system of the abovepatents.

The positioning of the conductor 32 in the case of the electrodes 30,31, and the conductor in case of electrodes 13 and 141 on the outersurface of the drill collar, at least until they pass above the locationof the other electrodes 30 and 14, positioned on the interior of thedrill collar, substantially eliminates the interaction of the magneticfields of the conductor and the drill collar, which otherwise cause theelectrical impedances between the respective pairs of electrodes to beextremely small. The exact reason for this is not fully understood, butactual tests have proven that the above system will operate successfullywhile a system wherein the conductors from both electrodes are disposedin the interior of the drill collar will not operate. Of course, it ispossible to reverse the position of the electrodes shown in FIG. 1 andposition the lower electrodes on the inner surface of the drill collarand the upper electrodes on the exterior of the drill collar and stillobtain satisfactory results, providing the conductors from the lowerelectrodes are mounted on the exterior of the drill collar.

Referring to FIG. 3 there is shown the logging system of the presentinvention adapted to a conventional electrical logging tool with thelogging tool disposed within a borehole 60. The logging tool 61comprises a hollow elongated cylindrical member 61 having a vent opening62 at the bottom and a similar vent opening 63 at the top.

The vent openings permit the borehole fluid to circulate freely throughthe logging tool. The member 61 should be formed of ferromagneticmaterial for example steel or iron. The tool includes a pair of detectorelectrodes 65 and 66 with the electrode 65 mounted on the outer surfaceand electrode 66 on the inner surface. Similarly the lower or outerelectrode 65 is coupled by means of a conductor 70 disposed on the outersurface of the tool to the logging instrumentation. The conductor 71)passed up the outer surface of the tool until it is above the locationof the inner electrode 66 where it then passes through pressure seal'71. A similar pair of source electrodes 72 and 73 are disposed at thetop of the tool for producing a current flow in the formationsurrounding the borehole. The logging tool is connected to a loggingcable 741 which in addition to furnishing the mechanical support forsupporting the tool in the borehole also includes the requiredelectrical circuits for transmitting power to the tool and informationsignals from the tool to the surface.

The logging tool shown in FIG. 3 operates in the same manner as thedrill collar tool shown in FIGS. 1 and 2. The logging tool shown in FIG.3 has the advantage over previously used electrical logging tools inthat the source electrodes will induce a vertical current flow in theformation in place of the circular current flow produced by inductionlogging tools, but will possess the desirable feature of inductionlogging tools in that it will cause no current flow in the mud or in theformation immediately adjacent thereto. Thus, the resistivitymeasurements will be related to resistivity over vertical sections ofthe formation.

In addition to the above features of this invention, it is also possibleto use the resistivity tool as a direction seeking tool. For example, ifone is drilling a borehole adjacent to a previously drilled boreholewhich is cased with a ferromagnetic liner, it is possible to tell thedirection of the previously cased borehole from the second borehole. Forexample, when electrodes 30 and 31 of FIG. 1 are closest to the casedborehole, the resulting signal will have a minimum amplitude. Thedirection seeking ability of the present resistivity logging tool wouldbe extremely useful where multiple wells are drilled from an offshoreplatform. In this case, it is necessary to know the direction andapproximate distance to the previously drilled wells in order that thewell being drilled will bypass the previous wells. Many instances occurwherein a later drilled well intersects or drills through the casing ofa previously drilled encased well. When this occurs, it is possible tohave serious problems such as blowouts and the like.

We claim as our invention:

1. An electrical logging system comprising:

an elongated ferromagnetic member disposed to be lowered in a borehole;

a pair of source electrodes adapted to be connected to a high frequencysource of electrical energy, said source electrodes being disposed onthe surface of said ferromagnetic member and making electrical contactwith said ferromagnetic member;

a pair of detecting electrodes for detecting a voltage drop in theelongated ferromagnetic member, said pair being disposed on the surfaceof said ferromagnetic member and making electrical contact with saidferromagnetic member;

means for coupling said source and detecting electrodes to the surfaceof said borehole, said means including at least one conductor for eachpair of electrodes, each of said at least one conductor being disposedon the outer surface of said ferro- 4. The system of claim 1 whereinsaid member comprises a drill collar having a drill bit attached to itslower end, said source electrodes being mounted on the surface of thedrill collar above the location of said pair of detector electrodes.

5. The system of claim 4 where said source electrodes are excited by asource of high frequency alternating voltage having a frequencyexceeding ten kilohertz.

1. An electrical logging system comprising: an elongated ferromagneticmember disposed to be lowered in a borehole; a pair of source electrodesadapted to be connected to a high frequency source of electrical energy,said source electrodes being disposed on the surface of saidferromagnetic member and making electrical contact with saidferromagnetic member; a pair of detecting electrodes for detecting avoltage drop in the elongated ferromagnetic member, said pair beingdisposed on the surface of said ferromagnetic member and makingelectrical contact with said ferromagnetic member; means for couplingsaid source and detecting electrodes to the surface of said borehole,said means including at least one conductor for each pair of electrodes,each of said at least one conductor being disposed on the outer surfaceof said ferromagnetic member for a distance substantially equal to thedistance between the pair of electrodes.
 2. The system of claim 1 and inaddition means for rotating said member.
 3. The system of claim 2wherein said member is a portion of a drill string and one of saiddetecting electrodes is mounted on the outer surface of the drill stringand both of said detecting electrodes are longitudinally spaced belowsaid source electrodes.
 4. The system of claim 1 wherein said membercomprises a drill collar having a drill bit attached to its lower end,said source electrodes being mounted on the surface of the drill collarabove the location of said pair of detector electrodes.
 5. The system ofclaim 4 where said source electrodes are excited by a source of highfrequency alternating voltage having a frequency exceeding tenkilohertz.